This invention relates to an improved Fresnel lens, as well as to a method and mold for manufacturing it.
It is known to use, in order to cut down the weight and cost of wide aperture lenses, so-called Fresnel lenses which comprise a central lenticular portion surrounded by concentrically arranged prismatic rings. In actual practice, Fresnel lenses of this general type are usually provided with a first smooth face, of either flat or conical or frustum or spherical configuration, and second face formed with a series of saw-like teeth encircling a centrally located core lens. These teeth extend across their related Fresnel lens face as a set of concentrical rings comprising a refractive portion, which has an inclination arranged to vary from one tooth to the next, or between concentrical rings, and a connection slope or flank in the form of a steeply inclined front arranged to interconnect the refractive portion of one tooth with the refractive portion of an adjacent tooth.
The optical quality of Fresnel lenses essentially depends on the shape of said teeth, and in particular of said connection slopes or flanks which--performing no optical functions, but merely that of interconnecting two consecutive refractive portions together--are required to interfere as little as possible with the light path. In ideal technical conditions, said connection slopes or steep fronts of the teeth should extend substantially parallel to the direction of the light beam which is being transmitted through the lens, so as to induce the least possible disturbance. However, this optimum situation can not be achieved in actual practice, where Fresnel lenses of conventional design are manufactured with molding techniques, utilizing materials with a suitable refractive index. In fact, molding implies design limitations arising from mold withdrawal requisites, so that said steep front must be suitably located for withdrawal from the mold. Actually, the aforesaid steep front or connection slope is prevented from following the true light path through thus produced Fresnel lenses, and happens to interfere with said light path, so that optical performance is affected.
The above situation is further aggravated by the unavoidable presence of radiused inner and outer corner edges on said teeth, which further reduces the useful refractive portions.